The human and the tree: a love story

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Photo by Phillip Sauerbeck

I suspect no other relationship is more complex and fraught than that between humans and trees. I’ve been wanting for a long time to write something about it, but every time I try, I get overwhelmed. Where to begin?

For us humans, it indeed goes back to the beginning: Adam and Eve learned of their own humanity from a tree. Or if you prefer more scientific stories, our ancestors took a crucial step in speciating from other apes by descending from the trees. Since then we haven’t gone far from the tree, so to speak. We have eaten from trees, climbed trees, lived in trees, worshipped trees, studied trees, planted trees, bred trees, hugged trees, and saved trees. We have also, at various times, cut trees down for fuel, for lumber, to make paper, to make weapons, to clear farmland, to create subdivisions, because they threatened our infrastructure, because we didn’t like where they were growing, and for no reason whatsoever.

After hundreds of thousands of years of shared history, have we and trees come to understand each other better? Three stories I have come across recently suggest the answer is, it’s still complicated.

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The Story of the Constant, Changing Forest

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An olive tree from Ithaca, Odysseus’s home

If you’re a writer looking for a good symbol, consider the tree. The author of Genesis did, twice: he placed the tree of life and the tree of knowledge front and center in the Garden of Eden. Homer did, too: When his hero Odysseus returned home after twenty years of war and travel, needing to prove his identity to his skeptical wife, Penelope, he used a tree. “Move our bed into the hallway,” Penelope told her servant, laying a trap. (I’m paraphrasing here.) “It can’t be done,” Odysseus protested. “I carved a post of that bed from a living olive tree.” Only then did Penelope believe the strange man was really her husband, as steady as that post.

Trees have long impressed us with their steadfastness; in fact, some trees from Biblical times are still with us today. But a new story I read recently casts trees in a different role. I first came across a version of this story in a paper published in the journal BioScience in 2007. The authors looked at where trees live using a tool known as a “climate envelope,” which is a line drawn on a map around the entire range where a given species is able to survive. The scientists compared climate envelopes for 130 trees under 2007 conditions to those predicted for the end of the century, using the same computer models that the UN’s Intergovernmental Panel on Climate Change bases its forecasts on. On average, they found that trees’ envelopes moved 700 kilometers north, nearly the distance from Memphis to Chicago.

So does that mean our trees will be moving north as things get warmer? Traveling trees can make great stories: Shakespeare’s Macbeth was vanquished when Birnam Wood moved a few miles to his fortress at Dunsinane Hill. And it would be dramatic indeed if future northern woodsmen and women hunt deer among sprawling live oaks and big-leaf magnolias instead of spruce and pine trees. But the scientists who wrote the BioScience paper noted that actual trees are unlikely to track their climate envelopes’ northward migration in the coming years, at least if unassisted by humans. Trees can “move” up to a few miles in a generation, by setting their seeds aloft in the wind or encasing them in a shell so they can survive a trip in the gut of an animal. But tree generations are long, and most seeds land close to home. Sugar maples, for example, lead a chaste adolescence, and don’t start making seeds until the age of 22 or so. They then send out seeds attached to little helicopters, which spin and float at most the length of a football field before touching down. Scientists estimate trees’ maximum migration rate to be around 50 kilometers per century, with many traveling far slower—a tortoise’s pace in this race.

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